U.S. patent number 7,436,834 [Application Number 11/552,797] was granted by the patent office on 2008-10-14 for efficient frame retransmission in a wireless communication environment.
Invention is credited to Mark Earnshaw, Eman A Fituri, Bassam M. Hashem, Hang Zhang.
United States Patent |
7,436,834 |
Zhang , et al. |
October 14, 2008 |
**Please see images for:
( Certificate of Correction ) ** |
Efficient frame retransmission in a wireless communication
environment
Abstract
A multiplexer function maintains a transmission table of all
link control frames sent to a physical layer for transmission to a
receiver. The transmission table includes a list of frames from
various link control entities in the order in which they were
multiplexed together to form a stream of frames. At the receiver,
when a frame is detected that is in error, the physical layer sends
an indication to its associated demultiplexer function, which sets
a timer. When the timer expires, the demultiplexer function issues
a message, such as a negative acknowledgement message, directly to
the sender via the physical layer. The message includes information
about the link control frames preceding and following the errant
frame. The identified frames may be associated with different link
control entities, and are preferably provided in the order in which
they were received. The multiplexer function compares the
identified frames in the message to the list of frames in the
transmission table to identify the errant frame or frames and
operates to effect retransmission of the errant frame.
Inventors: |
Zhang; Hang (Nepean,
CA), Hashem; Bassam M. (Nepean, CA),
Earnshaw; Mark (Nepean, CA), Fituri; Eman A
(Nepean, CA) |
Family
ID: |
37480671 |
Appl.
No.: |
11/552,797 |
Filed: |
October 25, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
10108577 |
Mar 28, 2002 |
7145889 |
|
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Current U.S.
Class: |
370/389; 370/394;
370/469 |
Current CPC
Class: |
H04L
1/1671 (20130101); H04L 1/1887 (20130101); H04L
1/189 (20130101); H04W 48/08 (20130101); H04W
72/0406 (20130101) |
Current International
Class: |
H04L
12/26 (20060101) |
Field of
Search: |
;370/389,394,469 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Marcelo; Melvin
Attorney, Agent or Firm: Withrow & Terranova, PLLC
Parent Case Text
This application is a Divisional of U.S. patent application Ser.
No. 10/108,577 filed Mar. 28, 2002, currently pending, the
disclosure of which is incorporated herein by reference in its
entirety.
Claims
What is claimed is:
1. A wireless communication system comprising: a) a physical layer
facilitating wireless communications with a remote device over a
wireless medium; b) a link control layer with a plurality of link
control entities facilitating communication using a link control
protocol; c) a multiplexer function adapted to: i) multiplex frames
from the plurality of link control entities to create a stream of
frames, which are delivered to the physical layer for transmission
over the wireless medium to the remote device; ii) maintain a
transmission table containing transmitted frames; iii) receive an
indication from the remote device that at least one of the
transmitted frames was lost or corrupted and information regarding
ones of the transmitted frames that were properly received by the
remote device; and iv) identify the at least one of the transmitted
frames that was lost or corrupted by comparing the transmitted
frames of the transmission table with the ones of the transmitted
frames that were properly received by the remote device.
2. The system of claim 1 wherein the multiplexer function is
further adapted to effect retransmission of the at least one of the
transmitted frames that was lost or corrupted.
3. The system of claim 1 wherein the indication from the remote
device is a negative acknowledgement provided as part of an
automatic repeat request process.
4. The system of claim 1 wherein the multiplexer function is
further adapted to: a) receive alternate indication from the remote
device identifying at least one specific frame that was partially
corrupted during transmission when the remote device can determine
the identity of a partially corrupted frame; and b) effect
retransmission of the at least one specific frame that was lost or
corrupted.
5. A wireless communication system comprising: a) a physical layer
facilitating wireless communications with a remote device over a
wireless medium; b) a link control layer with a plurality of link
control entities facilitating communication using a link control
protocol; c) a demultiplexer function adapted to: i) demultiplex a
stream of frames recovered by the physical layer and deliver the
frames to corresponding ones of the plurality of link control
entities; ii) receive a message from the physical layer that at
least one of the frames was lost or corrupted; and iii) effect
transmission of an indication to the remote device that at least
one of the transmitted frames was lost or corrupted and information
regarding ones of the transmitted frames that were properly
received.
6. The system of claim 5 wherein the indication from the remote
device is a negative acknowledgement provided as part of an
automatic repeat request process.
7. The system of claim 5 wherein the demultiplexer function is
further adapted to effect transmission of an alternate indication
identifying at least one specific frame that was partially
corrupted during transmission when the identity of a partially
corrupted frame is determined.
8. The system of claim 5 wherein the demultiplexer function is
further adapted to: a) set a timer when the message from the
physical layer that at least one of the frames was lost or
corrupted is received; and b) when the timer expires, effect
transmission of the indication to the remote device that at least
one of the transmitted frames was lost or corrupted and information
regarding ones of the transmitted frames that were properly
received.
9. A method for facilitating retransmission comprising: a)
multiplexing frames from a plurality of link control entities to
create a stream of frames, which are delivered to a physical layer
for transmission over a wireless medium to a remote device; b)
maintaining a transmission table containing transmitted frames at a
multiplexer function; c) receiving an indication from the remote
device that at least one of the transmitted frames was lost or
corrupted and information regarding ones of the transmitted frames
that were properly received by the remote device; and d)
identifying the at least one of the transmitted frames that was
lost or corrupted by the comparing the transmitted frames of the
transmission table with the ones of the transmitted frames that
were properly received by the remote device at the multiplexer
function.
10. The method of claim 9 further comprising effecting
retransmission of the at least one of the transmitted frames that
was lost or corrupted via the multiplexer function.
11. The method of claim 9 wherein the indication from the remote
device is a negative acknowledgement provided as part of an
automatic repeat request process.
12. The method of claim 9 further comprising: a) receiving an
alternate indication from the remote device identifying at least
one specific frame that was partially corrupted during transmission
when the remote device can determine the identity of a partially
corrupted frame; and b) effecting retransmission of the at least
one specific frame that was lost or corrupted via the multiplexer
function.
13. A method for facilitating retransmission comprising: a)
demultiplexing a stream of frames recovered by a physical layer and
delivering the frames to corresponding ones of a plurality of link
control entities; b) receiving a message from the physical layer
that at least one of the frames was lost or corrupted; and c)
effecting transmission of an indication to a remote device that at
least one of the transmitted frames was lost or corrupted and
information regarding ones of the transmitted frames that were
properly received via a demultiplexer function.
14. The method of claim 13 wherein the indication from the remote
device is a negative acknowledgement provided as part of an
automatic repeat request process.
15. The method of claim 13 further comprising effecting
transmission of an alternate indication identifying at least one
specific frame that was partially corrupted during transmission
when the identity of a partially corrupted frame is determined.
16. The method of claim 13 further comprising: a) setting a timer
when the message from the physical layer that at least one of the
frames was lost or corrupted is received; and b) when the timer
expires, effecting transmission of the indication to the remote
device that at least one of the transmitted frames was lost or
corrupted and information regarding ones of the transmitted frames
that were properly received.
Description
FIELD OF THE INVENTION
The present invention relates to wireless communications, and in
particular to facilitating faster and more efficient retransmission
of lost or corrupt data.
BACKGROUND OF THE INVENTION
Many wireless communication systems assign transmission resources
using one or more schedulers associated with a base station, which
serves multiple mobile terminals. Typically, these resources have a
time division multiple access (TDMA) component, wherein
communications between the base station and a select mobile
terminal are assigned to a given time slot. For downlink
communications wherein the base station transmits data to the
mobile terminal, the base station's downlink scheduler receives
data for transmission to the mobile terminal, and allocates a
certain time slot in which to transmit data to the mobile terminal.
Notably, transmissions to different mobile terminals are assigned
to different time slots to facilitate an ordered transmission of
data amongst the mobile terminals. Similarly, in uplink
communications wherein the mobile terminal transmits data to the
base station for delivery across the network, the base station's
uplink scheduler determines when the mobile terminal can transmit
information to the base station, and via control signaling,
instructs the mobile terminal of the time slots in which to
transmit data to the base station.
Typically, packets of data are sequentially transmitted between the
base station and the mobile terminal in the form of frames, which
may be lost or corrupted due to channel conditions during
transmission. The major objectives of layer 2 protocols in wireless
access networks are to perform resource sharing among multiple
services and multiple users and to provide improved radio link
quality and reliability by implementing a retransmission mechanism
for lost or corrupted frames associated with non-delay-sensitive
services and applications. For CDMA standard, the radio link
protocol (RLP) uses an automatic repeat request (ARQ) protocol to
monitor incoming frames and request retransmission of lost or
corrupted frames. An Internet Protocol (IP) based RLP design allows
an RLP frame to encapsulate an IP packet or fragment of an IP
packet. Each RLP frame header includes a sequence number to
maintain the integrity of RLP frames flowing over the wireless
link. In a negative acknowledgment (NAK) based RLP ARQ scheme,
after identifying the loss of an RLP frame at the receiver RLP
entity, a NAK is sent to the transmitter RLP entity. The NAK
identifies the lost RLP frame and triggers retransmission of the
lost RLP frame by the transmitter RLP entity. Lost RLP frames are
determined by checking the sequence numbers of subsequently
received RLP frames. Once an RLP frame is lost, a significant
amount of time may pass before receiving a subsequent RLP frame,
which is capable of providing information to determine that the
previous RLP frame was lost.
For example, the reception of RLP frames N and N+2 in a row
indicates that RLP frame N+1 was lost. After receiving RLP frame
N+2, the receiver RLP entity sends a NAK request indicating a frame
was lost. In a high-speed wireless Internet access system, arrival
times for frames often vary greatly due to the high non-stream-like
nature, or burstiness, of packet applications. If frame N+2 arrives
at the receiver a relatively long time after RLP frame N, then the
receiver RLP layer will take a longer period of time to identify
the possible loss of RLP frame N+1. The result is a longer wireless
link delay for RLP frame N+1.
In a traditional RLP scheme, retransmissions are under control of
the transmission RLP entity. That is, the receiver RLP entity is
only responsible for informing the transmission RLP entity that a
given RLP frame was lost or received in error. The transmission RLP
entity is responsible for determining when to retransmit that RLP
frame and how many copies to include based on requisite
retransmission parameters. The retransmission parameters are
primarily a function of the QoS level associated with the data
stream being considered. For example, the retransmission parameters
may relate to acceptable error rates and transmission delays.
As noted above, the base station controls resources for uplink
communications. If the mobile terminal does not have any extra
transmission resources to use for retransmission purposes, then the
mobile terminal's RLP entity would need to request additional
transmission resources from the base station to perform any
required retransmissions. When an RLP frame is lost, the base
station's RLP entity sends a NAK to the mobile terminal's RLP
entity. The mobile terminal has to determine what additional
transmission resources are required for retransmission and then
request those resources from the base station via a transmission
request. Under control of the uplink scheduler, the base station
must grant the additional resources for the mobile terminal,
schedule transmission of the transmission grant, and transmit the
transmission grant to the mobile terminal. Upon receipt of the
transmission grant, the mobile terminal's RLP entity is then
finally able to initiate retransmission of the frame and any copies
thereof. The process involves an extra round of signaling between
the mobile terminal and the base station, thereby increasing both
the delay due to retransmission and the amount of signaling
overhead carried on both uplink and downlink channels.
Accordingly, the traditional RLP schemes incur additional delays
and signaling overhead due to the need for the mobile terminal's
RLP entity to request additional transmission resources from the
base station's uplink scheduler for each retransmission. The
additional signaling delays reduce the QoS levels for uplink
communications. Further, the extra signaling overhead will likely
have a negative impact on total system capacity, since it will
reduce the amount of user data that can be transmitted over both
the uplink and downlink. As such, there is a need to minimize the
time required to identify lost or corrupt RLP frames and to
decrease the delay in retransmitting the lost or corrupt RLP frames
without sacrificing reliability.
SUMMARY OF THE INVENTION
The present invention relates to a link control automatic repeat
request (ARQ) operation in a wireless communication environment.
When a terminal sends an original request for communication
resources, the request includes not only payload size, but
preferably quality of service parameters associated with the data
to be transmitted. During uplink communications, the base station
performs the link control based ARQ for lost or corrupt packets.
When lost or corrupt packets are identified, a link control entity
at the base station determines the appropriate retransmission
parameters and contacts the uplink scheduler, if necessary, to
obtain the corresponding communication resources for
retransmission. The additional communication resources for
retransmission are provided in a message, such as a negative
acknowledgement message, which is sent to the mobile terminal to
trigger retransmission of the lost or corrupted data.
The link control ARQ operation may switch between traditional ARQ
operation wherein the mobile terminal controls retransmission, and
that of the present invention wherein the base station controls
retransmission, depending on whether sufficient communication
resources have been previously allocated to the mobile terminal.
Since the base station is aware of the quantity of communication
resources that have already been granted to the mobile terminal,
the base station can dynamically switch between the retransmission
modes.
Accordingly, when retransmission is required, the base station
checks with the uplink scheduler to determine the availability of
the communication resources already assigned to the mobile
terminal. If the previously assigned resources are sufficient for
retransmission, the base station may issue a simple negative
acknowledgement without the provisioning of additional
communication resources for retransmission. Otherwise, the base
station will request additional communication resources from the
uplink scheduler and grant the additional communication resources
via the negative acknowledgement message.
In another embodiment of the present invention, a multiplexer
function of the sender maintains a transmission table of all link
control frames sent to a physical layer for transmission to a
receiver. The transmission table includes a list of frames from
various link control entities in the order in which they were
multiplexed together to form a stream of frames. At the receiver,
when a link control frame is detected that is in error, the
physical layer sends an indication to its associated demultiplexer
function, which sets a timer. When the timer expires, the
demultiplexer function issues a message, such as a negative
acknowledgement message, directly to the sender via the physical
layer. The message includes information about the link control
frames preceding and following the errant frame. The identified
frames may be associated with different link control entities, and
are preferably provided in the order in which they were
received.
The multiplexer function of the sender compares the identified
frames in the message to the list of frames in the transmission
table to identify the errant frame or frames. Once the errant frame
is identified, the multiplexer function will operate to effect
retransmission of the errant frame. The multiplexer function may
directly send the frame, or may contact the associated RLP entity
to trigger immediate retransmission.
If the errant frame is only partially corrupted, wherein the
receiver can determine the identity of the frame but not its
contents, the demultiplexer function may notify the corresponding
link control entity to trigger a negative acknowledgement message
or the like, specifically identifying the errant frame. This
embodiment is applicable to uplink and downlink communications.
Those skilled in the art will appreciate the scope of the present
invention and realize additional aspects thereof after reading the
following detailed description of the preferred embodiments in
association with the accompanying drawing figures.
BRIEF DESCRIPTION OF THE DRAWING FIGURES
The accompanying drawing figures incorporated in and forming a part
of this specification illustrate several aspects of the invention,
and together with the description serve to explain the principles
of the invention.
FIG. 1 is block representation of a communication environment
including a base station, base station controller, and mobile
terminal.
FIG. 2 is logical representation of the base station and mobile
terminal according to one embodiment of the present invention.
FIG. 3 is an RLP retransmission flow according to the prior
art.
FIG. 4 is an RLP retransmission flow according to a first
embodiment of the present invention.
FIG. 5 is another RLP retransmission flow according to the prior
art.
FIG. 6 is an RLP retransmission flow according to a second
embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The embodiments set forth below represent the necessary information
to enable those skilled in the art to practice the invention and
illustrate the best mode of practicing the invention. Upon reading
the following description in light of the accompanying drawing
figures, those skilled in the art will understand the concepts of
the invention and will recognize applications of these concepts not
particularly addressed herein. It should be understood that these
concepts and applications fall within the scope of the disclosure
and the accompanying claims.
With reference to FIG. 1, an exemplary wireless communication
environment is illustrated, wherein a base station controller 8 and
a base station 10 cooperate to facilitate communications between a
mobile terminal 12 and any number of devices on a network 14. In
general, the mobile terminal 12 includes receive circuitry 16 and
transmit circuitry 18 for facilitating downlink and uplink
communications, respectively, with the base station 10. The mobile
terminal 12 will also include a control system 20 having resident
application programs 22, which are capable of processing incoming
information and providing outgoing information for transmission to
the base station 10 via the transmit circuitry 18.
In one embodiment, the base station controller 8 will control a
number of associated base stations 10 and act as an interface to
the network 14, which may be a packet network, such as the
Internet. Although the described embodiments allocate significant
functionality in the base station 10, much of the functionality can
also be allocated to the base station controller 8 or shared
between the base station 10 and the base station controller 8. The
base station 10 may include transmit circuitry 24 and receive
circuitry 26 configured to facilitate wireless communications with
the mobile terminal 12. The transmit and receive circuitry 24, 26
cooperate with a network interface 28, a downlink scheduler 30, and
an uplink scheduler 32 to schedule communications between the
mobile terminal 12 and the network 14, via the base station
controller 8. In general, the thick, solid lines indicate dataflow;
the thin, solid lines indicate control and signaling information;
and the dashed lines indicate wireless communications, which may
include data control and signaling information.
For downlink communications, data received directly or indirectly
from the network 14 at the network interface 28 is delivered to the
downlink scheduler 30 (line 34) for scheduling. Notably, data is
continuously received for a variety of mobile terminals 12 that are
served by the base station 10, and the downlink scheduler 30 will
schedule the data to be transmitted to the mobile terminal 12
during a given time slot within a scheduling period. Once
scheduled, the data is systematically provided to the transmit
circuitry 24 (line 36) for transmission to the various mobile
terminals 12 during a time period corresponding to the assigned
time slot (line 38).
As those skilled in the art will appreciate, the transmitted data
is attached with a user identification or other equivalent method
such that the mobile terminal 12 will only receive and recover
information intended to be transmitted and ignore information
intended for other mobile terminals 12. Accordingly, the receive
circuitry 16 of the mobile terminal 12 will receive, downconvert,
and demodulate the transmitted signals to uncover the transmitted
data, which is then sent to the proper application program 22 of
the control system 20 (line 40).
For uplink communications, the application program 22 will generate
data for delivery over the packet network 14. The control system 20
will deliver the data to be transmitted to the mobile terminal's
transmit circuitry 18 (line 42), which will modulate and transmit
the data to the base station 10 (line 44). The receive circuitry 26
of the base station 10 will downconvert, demodulate, and recover
the transmitted data, which will be delivered to the network
interface 28 (line 46) for delivery to the packet network 14.
Notably, the uplink and downlink communications (lines 38 and 44)
also include control signaling and other information to facilitate
reliable communications between the mobile terminal 12 and the base
station 10. A significant amount of control signaling between the
mobile terminal 12 and the base station 10 relates to scheduling
uplink transmissions from the mobile terminal 12 to the base
station 10. For example, the mobile terminal 12 typically transmits
only during certain time slots in an uplink scheduling period,
wherein the base station 10, via downlink communications (line 38),
tells the mobile terminal 12 which slots to use for uplink
communications (line 44). Accordingly, the mobile terminal 12 will
send transmission requests to the base station 10. The transmission
requests are received and processed in the receive circuitry 26 and
sent to the uplink scheduler 32 (line 48).
The uplink scheduler 32 will process the transmission requests from
the mobile terminal 12 and provide transmission grants for the
mobile terminal 12. The transmission grants authorize the mobile
terminal 12 to transmit for uplink communications during defined
time slots. The transmission grants must be sent to the mobile
terminal 12 via downlink communications (line 38), and thus are
sent to the downlink scheduler 30 (line 50), which will schedule
delivery of the transmission requests along with the data being
transmitted to the various mobile terminals 12 as described above.
The mobile terminal 12 will receive the transmission grant and
transmit the data subject to the transmission request during the
time slot or slots authorized by the base station 10.
The base station 10 must provide transmission grants for both data
being transmitted and data being retransmitted because the
originally transmitted data was lost. Packets of data are
sequentially transmitted between the base station 10 and the mobile
terminal 12 in the form of frames, which may be lost or corrupted
due to channel conditions during transmission. Accordingly, when
the receive circuitry 16 of the mobile terminal detects that a
frame has been lost or corrupted, the transmit circuitry 18 of the
mobile terminal 12 is signaled of the loss (line 52) and operates
to send information bearing on the lost or corrupted frame to the
receive circuitry 26 of the base station 10 (line 44). As with
original transmissions, retransmission may require an additional
allocation of communication resources. Accordingly, the uplink
scheduler 32 must schedule resources for retransmission and
cooperate with the downlink scheduler 30 (line 50) to transmit a
transmission grant to the mobile terminal 12.
FIG. 2 illustrates an architecture having cooperating link control
entities in the mobile terminal 12 and base station 10 of a
wireless communication system according to one embodiment of the
present invention. Notably, the logical functionality of a control
system for the base station 10 can be distributed to any degree
among the base station 10 and the base station controller 8. For
conciseness and readability, the description assigns most of the
functionality of the present invention to the base station 10. Both
the base station 10 and the mobile terminal 12 will support
applications and layer 3 signaling services and upper layer
services. In one embodiment, the layer 2 link control protocol is
preferably the Radio Link Protocol (RLP) provided by an RLP entity
56 for the base station 10 and an RLP entity 58 for the mobile
terminal 12. The base station RLP entity 56 normally assigns an RLP
entity 60, RLP 1 through RLP N, for each layer 3 data services
entity 52 and for each user, user 1 through user N, requiring
communication capability. Similarly, the mobile terminal RLP layer
58 assigns an RLP entity 62, RLP 1 through RLP N, for each layer 3
application entity 54 requiring communication capability.
For transmission, each RLP entity 60, 62 will encapsulate all or a
portion of one or more packets from the layer 3 applications and
protocols 52, 54 into RLP frames. During reception, each RLP entity
60, 62 will process the incoming RLP frames to recover the one or
more packets for the layer 3 entity 52, 54. Further, each RLP
entity 60, 62 is associated with an automatic repeat request (ARQ)
function 64, 66 for the base station 10 and mobile terminal 12,
respectively. The ARQ functions 64, 66 control retransmission of
lost or corrupted RLP frames. Each ARQ function 64, 66 is
associated with a data buffer 70, 72 for buffering RLP frames being
transmitted and a retransmit buffer 74, 76 for storing RLP frames
that have been transmitted in case retransmission is required.
Both the base station 10 and mobile terminal 12 include multiplexer
(MUX) functions 78, 80 used to multiplex RLP frames received from
the various applications of the associated users for transmission.
The multiplexer functions 78, 80 create media access control (MAC)
frames, which encapsulate all or a portion of one or more RLP
frames, and provide the MAC frames to the associated physical
layers 82, 84. The physical layers 82, 84 then create physical
layer frames and transmit the physical layer frames over a wireless
medium 86.
During reception of transmitted physical layer frames, the physical
layers 82, 84 recover the MAC frames, which are then sent to an
associated demultiplexer (DEMUX) functions 86, 88. The
demultiplexer functions 86, 88 demultiplex and deliver the
recovered RLP frames to the appropriate layer 2 RLP entity 60, 62
of the layer 2 RLP 56, 58. For each RLP entity 60, 62, the received
RLP frames are stored in a re-synchronization buffer 90, 92 in
cooperation with the associated ARQ function 64, 66. Since the MAC
frames, and thus the RLP frames, may be received out of order, the
re-synchronization buffer 90, 92 is used to place the received RLP
frames in proper order prior to delivering the data to the layer 3
entities 52, 54. The ARQ function 64, 66 monitors incoming data and
may request retransmission of lost or corrupted RLP frames.
With reference to FIG. 3, conventional RLP operation is described
for uplink communications. Initially, the RLP entity 62 of the
mobile terminal 12 will send a transmission request (TX RQ) to the
RLP entity 60 of the base station 10 requesting communication
resources to transmit frames A1-A3. The transmission request may
include frame or payload size, as well as QoS parameters, such as a
maximum allowable transmission delay and desired error rates. After
receiving the transmission request at the base station 10, the
uplink scheduler 32 will assign the necessary uplink communication
resources for the mobile terminal 12 to transmit RLP frames A1-A3,
and will initiate a transmission grant (TX GRANT) providing
sufficient information to the RLP entity 62 of the mobile terminal
12 to transmit RLP frames A1-A3 during defined time slots. The
assigned communication resources provided with the transmission
grant may or may not include additional resources for any required
retransmissions. Further, the transmission requests may be sent
over a low rate control channel or over a random access
channel.
Assume that RLP frames A1-A3 are transmitted, but RLP frame A2 is
lost or otherwise corrupted as represented by the dashed line.
Typically, ARQ based retransmission is controlled by the RLP entity
of the sender, which for uplink communications is the RLP entity 62
of the mobile terminal 12. Upon receiving RLP frame A3 after RLP
frame A1, the RLP entity 60 determines that the RLP frame A2 is
lost and retransmission of the lost RLP frame A2 is necessary. The
RLP entity 60 of the base station 10 sends back a NAK specifying
RLP frame A2 was lost after determining that RLP frames A1 and A3
were received. The NAK may simply provide the sequence number for
the lost RLP frame. After receiving the NAK, the RLP entity 62 of
the mobile terminal 12 performs ARQ by determining the appropriate
retransmission parameters based on the QoS required by the lost RLP
frame A2. Assume the RLP entity 62 of the mobile terminal 12
decides to send two copies of the lost RLP frame A2 during
retransmission. If insufficient communication resources are
available, the RLP entity 62 of the mobile terminal 12 must trigger
a second transmission request (TX RQ') for the additional
communication resources required for retransmission of the multiple
copies of RLP frame A2.
After receiving the transmission request at the base station 10,
the uplink scheduler 32 will assign the necessary uplink
communication resources for the mobile terminal 12 to transmit the
multiple copies of RLP frame A2, and the RLP entity 60 of the base
station 10 will initiate a second transmission grant (TX GRANT')
providing sufficient information to the RLP entity 62 of the mobile
terminal 12 to transmit RLP frame A2 during defined time slots.
After receiving the additional resources from the base station 10
via the second transmission grant, the RLP entity 62 of the mobile
terminal will retransmit multiple copies of the RLP frame A2. As
illustrated, when the assigned uplink communication resources for
initial transmission have insufficient capacity for retransmission
of lost RLP frames, the conventional mode is inefficient and
results in excessive retransmission delays.
The present invention provides a technique for minimizing the
retransmission delays when sufficient communication resources have
not been allocated for retransmitting lost RLP frames. With
reference to FIG. 4, the technique is described in light of the
example provided in FIG. 3. Initially, the RLP entity 62 of the
mobile terminal 12 will send a transmission request (TX RQ) to the
RLP entity 60 of the base station 10 requesting communication
resources to transmit frames A1-A3. Preferably, the transmission
request will include not only the payload size, but also any QoS
parameters necessary to allow the base station 10 to control
retransmission, if necessary. After receiving the transmission
request at the base station 10, the uplink scheduler 32 will assign
the necessary uplink communication resources for the mobile
terminal 12 to transmit RLP frames A1-A3, and the RLP entity 60 of
the base station 10 will initiate a transmission grant (TX GRANT)
providing sufficient information to the RLP entity 62 of the mobile
terminal 12 to transmit RLP frames A1-A3 during defined time
slots.
Again, assume that RLP frames A1-A3 are transmitted, but RLP frame
A2 is lost or otherwise corrupted. Upon receiving RLP frame A3
after RLP frame A1, the RLP entity 60 of the base station 10
determines that RLP frame A2 is lost and retransmission of the lost
RLP frame A2 is necessary. Instead of simply sending back a NAK
specifying RLP frame A2 was lost after determining that RLP frame,
the RLP entity 60 of the base station 10 will determine the
appropriate retransmission parameters based on the QoS required by
the lost RLP frame A2. The retransmission parameters may identify
the number of copies of the lost RLP frame to send and when to
retransmit based on the QoS parameters, such as the maximum
allowable delays associated with lost RLP frames or acceptable
error rates, provided with the original transmission request.
Assume the RLP entity 60 of the base station 10 decides that two
copies of the lost RLP frame A2 should be transmitted by the mobile
terminal 10 during retransmission. Next, the RLP entity 60 of the
base station 10 will contact the uplink scheduler 32 to obtain
communication resources required for retransmission of the lost RLP
frame A2 and any copies thereof. Preferably, the RLP entity 60 and
the uplink scheduler 32 will cooperate to determine if additional
communication resources are required for retransmission or if
sufficient communication resources for retransmission have already
been granted.
If there are insufficient resources to facilitate retransmission of
the lost RLP frame A2 and any requisite copies thereof, the uplink
scheduler 32 will provide additional communication resources to the
RLP entity 60 of the base station 10 for retransmission of the lost
RLP frame A2 and any copies thereof. The RLP entity 60 of the base
station 10 then sends back a NAK including identification of the
lost RLP frame A2, the retransmission parameters, and, if
necessary, a transmission grant for retransmission. The
transmission grant corresponds to the additional communication
resources allocated for retransmission. After receiving the NAK,
the RLP entity 62 of the mobile terminal 12 can immediately trigger
retransmission of the lost RLP frame A2 according to the
retransmission parameters provided by the RLP entity 60 of the base
station 10 and according to the communication resources allocated
by the uplink scheduler 32.
Accordingly, the RLP entity 60 of the base station 10 identifies a
lost RLP frame, and the base station 10 determines the appropriate
retransmission parameters based on the QoS levels and issues a NAK,
which includes the corresponding additional communication resources
needed for retransmission. The RLP entity 62 of the mobile terminal
12 is then able to quickly retransmit the RLP frames using the
newly granted communication resources. The signaling overhead and
retransmission delays associated with the conventional
retransmission techniques are greatly reduced.
The present invention also facilitates switching between the
conventional and proposed RLP retransmission techniques. When the
base station 10 determines that the previously assigned
communication resources are sufficient to handle any necessary
retransmissions, the conventional RLP ARQ mode is applied wherein
the RLP entity 60 of the base station 10 sends a NAK to the RLP
entity 62 of the mobile terminal 12 for a lost RLP frame, and the
RLP entity 62 of the mobile terminal 12 determines when and how
many copies of the lost RLP frame to retransmit. Otherwise, when
the currently assigned resources are insufficient to handle
retransmissions, the proposed RLP mode is applied. Since the RLP
entity 60 of the base station 10 is aware of the communication
resources already granted, identifying which RLP mode to use and
switching dynamically between the two modes is readily achievable.
Whenever the RLP entity 60 of the base station 10 needs to request
retransmission, it first checks with the uplink scheduler 32 to
determine the availability of the mobile terminal's
already-assigned communication resources. If these communication
resources are sufficient for the retransmission, the RLP entity 60
of the base station 10 will issue a simple NAK and not attach any
additional resource assignment. Otherwise, the RLP entity 60 of the
base station 10 will request additional communication resources
from the uplink scheduler 32 and attach a corresponding
transmission grant to the NAK signal. Dynamic switching between the
two RLP modes can occur seamlessly as required. This multi-mode
uplink RLP scheme improves overall performance by reducing the
amount of signaling overhead and overall transmission delays.
For downlink communications, the base station's transmit circuitry
24 typically transmits at a consistent power level for all mobile
terminals 12 over all scheduled time slots. Given varying channel
conditions and other variables affecting wireless communications,
the base station 10 and mobile terminal 12 cooperate to achieve
communications with a desired error rate. If error rates are too
high, quality of service levels drop to undesirable levels and the
amount of traffic increases due to the retransmission of lost
information. If error rates are too low for any given mobile
terminal 12, communication resources are wasted and data rates
could be increased to optimize efficiency of the system.
Since the base station 10 typically maintains a constant transmit
power during downlink communications, the communication resources
for downlink communications may pertain to scheduled time slots for
transmission and the link mode used to facilitate downlink
transmissions. The link mode relates to the type of coding,
modulation, and data rates used to facilitate downlink
communications. In general, the more robust the encoding and
modulation, the lower the data rates, wherein the goal would be to
achieve the desired error rate while maximizing data rate and using
higher encoding and higher order modulation.
For uplink communications, transmit power levels of the mobile
terminals 12 are constantly varied in an effort to maintain the
desired error rates for uplink communications. Keeping the mobile
terminal's transmit power at minimum required levels to achieve a
desired error rate minimizes the interference injected on
communications associated with other mobile terminals 12, as well
as extending the battery life of the mobile terminal 12. Thus, the
primary communication resources requested by the mobile terminals
12 and granted by the base station 10 for uplink communications are
time slots and the transmit power required to achieve the minimum
error rate for uplink communications.
Another embodiment of the present invention efficiently identifies
missing RLP frames in a multiplexed stream of RLP frames
originating from two or more applications and users. As described
above, each layer 3 entity 52, 54 has its own RLP entity 60, 62,
which forms RLP frames by inserting a RLP frame header, including a
sequence number and possible fragmentation information. The RLP
frames originating from different applications 52, 54 are
encapsulated into MAC frames and are multiplexed together by
multiplexer function 78, 80 into a single data stream, which is
sent to the physical layer 82, 84. The physical layer 82, 84
creates physical layer frames from the stream of MAC frames and
transmits the physical layer frames over the wireless link. The MAC
frames typically include a MAC frame header having an application
ID and, for downlink communications, a user ID.
During reception, the physical layer 82, 84 receives the
transmitted physical layer frames, recovers the MAC frames, and
sends the MAC frames to the demultiplexer function 86, 88. The
demultiplexer function 86, 88 separates the RLP frames based on the
application, and if the RLP frames are being received by the base
station 10, further separates the RLP frames based on the user. The
demultiplexer function 86, 88 sends each RLP frame to the
corresponding RLP entity 60, 62. As noted, the RLP entity 60, 62
has traditionally been responsible for identifying any lost or
corrupted RLP frames and requesting retransmission.
A retransmission process according to the prior art is illustrated
for reference in FIG. 5. Although the concepts for this embodiment
are applicable to uplink and downlink communications, assume the
base station 10 is transmitting information to the mobile terminal
12 over a downlink and that three applications A, B, and C produce
a multiplexed data stream of RLP frames to be transmitted. Each
application produces information resulting in three RLP frames
A1-A3, B1-B3, and C1-C3. The resultant multiplexed data stream is
as follows A1, B1, C1, C2, A2, B2, C3, A3, and B3 wherein each
original transmission occurs as scheduled. For the sake of
simplicity, frame or packet fragmentation is not shown.
As depicted, RLP frame B1 is lost during transmission (100).
Traditionally, the RLP entity 62 of the mobile terminal 12 cannot
determine that RLP frame B1 was lost until RLP frame B2 is received
several time slots later (102). By the time retransmission for RLP
frame B1 is requested (104) and performed, a retransmission delay
of six time slots has been incurred. Upon receipt of the
retransmitted RLP frame B1, the RLP entity 62 will process RLP
frames B1 and B2 in traditional fashion (106). Note that the
retransmission delay is increased if RLP frame B2 is also lost or
its transmission delayed by additional multiplexed RLP frames from
the other applications 52.
FIG. 6 illustrates the multi-application RLP scheme according to
one embodiment of the present invention. Again, the concepts for
this embodiment are applicable to uplink and downlink
communications; however, assume the base station 10 is transmitting
information to the mobile terminal 12 over a downlink and that
three applications A, B, and C produce a multiplexed data stream of
RLP frames to be transmitted. Each application produces information
resulting in three RLP frames A1-A3, B1-B3, and C1-C3. The
resultant multiplexed data stream is as follows A1, B1, C1, C2, A2,
B2, C3, A3, and B3 wherein each original transmission occurs as
scheduled.
A transmission table 200 is associated with the multiplexer
function 78 and maintains a record of the RLP frames that have been
transmitted for all of the applications and users. Assume that RLP
frame B1 is corrupted during transmission (202). During reception,
the physical layer 84 of the mobile terminal 12 will receive the
transmitted physical layer frames and recover the encapsulated MAC
frames. The MAC frames are sent to demultiplexer function 88 for
demultiplexing the RLP frames for the corresponding RLP entities
62. The demultiplexer function 88 keeps track of properly received
RLP frames from the various applications.
The physical layer 84 is able to detect failed attempts to receive
a physical layer frame or recover an MAC frame and send an
indication of the error to the demultiplexer function 88. Upon
receiving the indication of the error, the demultiplexer function
88 will set a timer to impose a short delay in case a copy of the
corrupted RLP frame is transmitted or the RLP frame is subsequently
recovered (204). The short delay also allows for recovery of
subsequent RLP frames. If the RLP frame is recovered before
expiration, the timer is cancelled. Reference is made to
application Ser. No. 10/020,834, filed Dec. 13, 2001, entitled
PHYSICAL LAYER ASSISTED RETRANSMISSION, currently pending, which is
incorporated herein by reference, for additional information on
using the physical layer to detect corrupted frames.
If the timer expires, which indicates retransmission of the RLP
frame is required, the demultiplexer function 88 issues a NAK for
transmission to the base station 10 (206). The NAK includes
information about the RLP frames preceding and following the lost
RLP frame, even if those RLP frames are from different
applications. The multiplexer function 78 at the base station 10
will use the information in the NAK to check its transmission table
200, identify the lost RLP frame, and notify the corresponding RLP
entity 60 at the base station 10 to trigger immediate
retransmission of the lost RLP frame (208). The lost RLP frame is
identified by comparing the properly received RLP frames identified
in the information provided with the NAK and the list of RLP frames
kept in the transmission table 200. The lost RLP frame is then
retransmitted (210). Use of the demultiplexer 88 and multiplexer 78
to assist in RLP retransmission shortens the retransmission delay
of RLP frame B1 to only three time slots, which is down from a
delay of six time slots if existing RLP algorithms are used.
If the RLP frame is only partially corrupted wherein the RLP
header, which includes the RLP frame identifier, is detected
correctly and only the payload is in error, the demultiplexer
function 88 can notify the corresponding RLP entity 62 to
immediately issue a NAK identifying the corrupted RLP frame. Those
skilled in the art will recognize the applicability of the second
embodiment to both uplink and downlink communications. For uplink
communications, the functionality and flow are simply reversed from
that described immediately above.
Those skilled in the art will recognize improvements and
modifications to the preferred embodiments of the present
invention. For example, various types of link control protocols may
benefit from the present invention and be implemented to various
degrees in and between the base station and base station
controller. Further, any packets or frames described above may
represent multiple packets and frames or portions thereof in case
of fragmentation. All such improvements and modifications are
considered within the scope of the concepts disclosed herein and
the claims that follow.
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